Intermittent operation of DC, Direct Current, supplied loads is common in many electrical systems. Radar RF, Radio Frequency, transmitters are one example of such load. Semiconductors are commonly used in RF power transmitters today. Supply voltages have become lower compared to prior transmitters, using vacuum tubes or similar, due to properties for semiconductors. Consequently have currents become equally higher for the same amount of power.
Modern AESA, Active Electronically Scanned Array, radar systems are made up from multiple solid state transceivers each connected to an antenna element being a part of an array of antenna elements. DC/DC converters and voltage regulators, powered from a DC bus, are distributed to each transceiver or to a group of transceivers.
The radar transmits RF pulses at a repetitions rate often referred to as the Pulse repetition Frequency, PRF. Pulse length and repetitions rate depend on usage of the radar system. This RF pulse mode operation is transformed to pulsating loads in the DC supply system, which even may be reflected back to the power supply source. This causes energy losses as well as risks for decreased performance and interference with other electrical systems. Control problems in an AC generator due to pulsating power load, is one example of interference that has been experienced. Another issue that has been discussed repeatedly is the risks for mechanical wear and tear in the generator and associated gearbox when supplying pulse operated systems, e.g. AESA radars.
FIG. 1 shows a typical supply for an AESA radar transmitter. An AC/DC 10 converter supplies a DC bus 11 voltage to distributed DC/DC-converters 20 each supplying a transmitter PA 30, Power Amplifier, or a group of transmitter PAs 30 with regulated DC voltage. The PAs 30 transmits pulsed electromagnetic RF signals. Effects from this pulsed operation are reflected back to the DC supply and also potentially further back to the AC supply. The intermittent power demand makes the supply currents become pulse modulated. For low pulse repetition rates there is an apparent risk for current modulation to propagate even to the AC-supply source 40. Capacitors (not shown) are placed on the voltage supply close to the transmitter and on the output of the DC/DC-converter 20 in order to enable supply of the pulse current drawn from the transmitters.
Pulsed loads reflected from the transmitters at higher PRF (>5 kHz) could often be adequately filtered using passive filters (LC-filters). The filters required for filtering at lower PRF will become bulky and would in most cases require too much space.
A current regulation loop could then be included in the DC/DC converter in order to charge the capacitors with a limited current striving to produce a constant supply current being the average of the pulsed current. This adds complexity and the trade off between voltage regulation and current regulation performance is not that easy to manage. Especially when taking all load conditions—full range of PRF and variations of duty cycle—into account. More advanced regulation techniques could then be implemented in order to improve voltage regulation and still filtering out the pulse currents. Additional input signals may be required, where information on radar operation (PRF etc) is supplied in advance in order to manage voltage regulation. This adds complexity and for an AESA radar—may be including hundreds of transmitters—this could be hard to handle. A digital SW based system would probably be suitable. It is, as already mentioned, preferred to filter out pulse currents completely close to the load (transmitter). In a practical implementation is filtering often distributed along the power supply chain in several steps in order to handle proper supply for all conditions. Load current filtering at lower PRF is often handled in the central AC/DC-converter 10.
The result is that fairly high pulse currents are reflected quite far back in the distribution. This could have impact on overall performance, e.g. power efficiency, EMC.
There is therefore a need for an improved solution for filtering of pulsed loads, which solution solves or at least mitigates at least one of the above mentioned problems.